Calories to Kilowatts: Understanding Human Power

The Basics of Energy Transfer

If you’ve been following this Qore Performance article series, you already know powering the human body takes energy. We quantify this power requirement using the “watt”. The concept of power- measured in watts- is fundamental to understanding how Qore Performance products act as a rocket booster for the human body. Qore Performance products like ICEPLATE® act as a heat sink or source, drawing away or adding to our metabolic heat to augment human thermoregulation. 

Unfortunately, aside from electricians, engineers, and scientists, “watts” (abbreviated as “W”) isn’t a well-known measurement. In fact, understanding energy transfer in terms of watts can be quite confusing. In this article we break down watts in the context of our metabolism and thermal physiology to explain how Qore Performance products provide a superhuman thermoregulatory advantage. 

Charging an iPhone, Charging a Human

Here’s the bottom line: The rate of energy transfer is known as “power”. We measure power using watts. Power is therefore energy transferred over time.

Similar to how speed is combines distance over time (i.e., 80mph means you are travelling at a rate of 80 miles in one hour), watts combine the unit for energy – a joule (abbreviated “J”) – with time. In fact, 1 watt equals one joule (a unit of energy) transferred every second. Mathematically, we write this as 1W = 1J/s.

To explain, let’s start with something familiar: your phone charger. New iPhones come with a 20W – a “twenty watt” – charger. This measurement tells us how fast the charger can take energy (in the form of electricity) from the wall outlet and put it into the phone battery. Stated differently, a 20W phone charger transfers 20 joules every second into the phone battery.[1]

A quick digression: Your brain runs on about 20 watts of metabolic power- roughly the same rate a modern smartphone charger delivers electricity. In other words, the energy rate needed to keep your neurons firing is on the same order of magnitude as charging your iPhone!

So, a new iPhone can juice up at a rate of 20W. If you (like me) misplace phone chargers, you might purchase a cheap off-brand replacement with an approximately 10W rating. Compared with the (lost) 20W charger, the knock-off 10W charger transfers energy at only half the rate, taking twice as long to fully charge the phone. [2]

So, watts measure the rate of energy transfer, but what is energy? That’s tricky, as energy has many forms. A few examples include kinetic energy (energy of motion), chemical energy (energy stored in molecular bonds), and thermal energy (energy of the random motion of particles, experienced as heat). 

Critically, energy types can be converted between each other, something known as the First Law of Thermodynamics, without loss of energy overall. It’s because energy is convertible that we measure it in any form with the standard unit “joule”. It’s cool to think the energy in a chunk of uranium, your phone battery, the food we eat, the metabolic waste heat we produce, and energy from the latent heat of fusion in a frozen ICEPLATE® are measured the same way. 

Human Power

Let’s relate this concept to humans. Nutrition labels on food have a small disclaimer on the bottom that “2,000 calories a day is used for general nutrition advice”. Calories, like joules, are a unit of energy (1 Calorie = 4,184 joules). (An important nuance here: in typical use, what we call “Calories” are actually kilocalories- multiples of 1,000 calories. A Pepsi can has about 150,000 “small-c” calories, but we say “150 Calories” as shorthand. In this article, we use the common term calories when we actually mean kilocalories.)

It is only by convention that we refer to energy in food as “calories” instead of “joules”. A similar comparison is how, by convention, we use miles and inches in the U.S. instead of kilometers and centimeters; both describe distance. For our purposes, it’s enough to know that calories and joules both measure energy.

Human metabolism converts the stored chemical energy in the food we eat with the oxygen we breath to release energy in more useful forms, such as mechanical energy to move and thermal energy to maintain our body temperature. 2,000 calories burned over one day comes out to an average power use of 97 watts. Unsurprisingly, the baseline power requirement to keep us alive is around 100 watts. 

If you’re the type of person interested in Qore Performance, you probably aren’t sitting on the couch all day. You need more power than the baseline 100 watts required for basic metabolism. Work and exercise consume energy; the harder the task, the more energy required- so the more power consumed.

Human Power & Metabolism

For thermoregulation, the increased power required for anything beyond just laying around matters because most of the power we use doesn’t perform useful work! Humans are highly inefficient, converting, on average, only about 20% of calories burned into useful work. The remaining 80% is converted in metabolic heat. This is problematic, especially in hot environments, since intense activities requiring lots of power generate a whole lot of additional metabolic waste heat.

To explain metabolic waste heat, consider this: your metabolism and fire operate on the same principle. Both combine oxygen with organic matter (i.e., your most recent meal) and in the process release energy. While fire is uncontrolled, our metabolism allows us to channel a small fraction of the released energy (that 20% useful work) into doing things like running and lifting.

Beyond maintaining a core temperature around 98.6^oF, the remaining 80% of energy released as heat during metabolism is useless to us. As dictated by the second law of thermodynamics, entropy (i.e., the disorder) of any system can only increase. Waste heat represents energy at high entropy, making it difficult to convert into work. Unlike mechanical or electrical energy, heat tends to dissipate into the environment rather than do anything useful. Metabolic waste heat is the manifestation of this second law, and our body must work hard to get rid of it. 

A second digression: The fact that burning and metabolism operate using the same principle was proven by scientist and “Father of Modern Chemistry” Antoine Lavoisier. Using a guinea pig as test subject, Lavoisier asked: how does the body produce heat?

Lavoisier placed the guinea pig inside a calorimeter- a device designed to measure heat exchange. Surrounding the animal was a layer of ice insulated to prevent external interference. As the guinea pig respired and metabolized food, it released heat, melting the ice. By measuring the amount of melted ice, Lavoisier determined how much heat the (likely very uncomfortable) animal produced. He next measured how much oxygen the guinea pig consumed, noting it was replaced by CO2, demonstrating that respiration was a combustion-like process occurring within the body. To link this with the chemical reaction known as "fire", he reran the experiment replacing the animal with a lump of burning coal, noting that- just like with the guinea pig- the hot coal melted ice while consuming oxygen and producing CO2. 

Lavoisier’s experiment revealed the body’s heat production is directly tied to metabolic activity, a principle central to thermal physiology today. The work advanced our understanding of energy balance and highlighted the relationship between the environment, metabolism, and thermoregulation—concepts that still shape how we optimize human performance in extreme conditions. For his trouble (and because of his status as an aristocrat), Lavoisier would lose his head to the guillotine in the French revolution.

Qore Performance Power

In the context of Qore Performance products, heat transfers between a person and ICEPLATE® (or, in smaller quantities, an ICEFLASK™). A single ICEPLATE® has a cooling capacity (or, more scientifically, an energy transfer rate) of 70 watts when ice melts over a period of two hours. As we’ve explored above, this power value changes if energy transfer between the body and the ICEPLATE® is faster or slower than 2 hours. If all ice melts in one hour, the cooling power of the same ICEPLATE® doubles (~140 Watts), while melting over 4 hours produces an average of around 35 watts.[3]

This is because the amount of energy required to melt ice in an ICEPLATE® doesn’t change - 1.5 liters of ice (the amount in an ICEPLATE®) requires 501,000 joules, or 120 calories, to melt (remember, both units measure energy). This remains true if melting takes a minute or a day- time is irrelevant. 

But, again, power measure the rate of energy transfer. In the context of thermal regulation, it’s the rate of metabolic waste heat produced that matters. To clarify, let’s use an example.

Imagine you eat a 400-calorie donut and want to “burn off” those calories. You have a few options. First, you can just not eat until your body's baseline metabolism burns through 400 calories. Since our metabolic baseline is about 100 watts, this takes about five hours. Over five hours, releasing nearly 400 calories as metabolic heat doesn’t stress the body at all (indeed, unless you are in a relatively hot environment, it’s actually necessary to keep your core temperature from falling below 98.6^oF).

If instead you went for a walk to burn those calories off, an activity that takes about 250 watts, you’d burn those 400 calories in a little under 2 hours. 300 or so of those calories would be released as waste heat with about 100 calories of energy converted into movement. Except in hot conditions, most people can dissipate 300 calories of metabolic waste heat over 2 hours through sweating and convective heat transfer and may not experience much core temperature rise.

What if you wanted to run flat-out to burn those calories? Assuming you are fit enough to sustain a run maintaining 800 watts of effort, you’d burn those calories in just over 30 minutes! Your body will likely have difficulty transferring the 300 or so calories burned as waste heat in just 30 minutes, so your core temperature may start to rise, and you’ll certainly start to sweat!

Qore Performance products are designed to assist with the transfer of metabolic waste heat away from the body, helping us to maintain our optimum core temperature. Working safely in the heat is a fundamental problem of physics and energy flows. Instead of relying only on natural processes, Qore Performance products augment our ability to dissipate metabolic waste heat using products that allow direct conductive transfer of excess energy, allowing us to push hard and work longer. 

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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120^oF) to Arctic conditions in central Alaska (-42^oF).